1. Field of the Invention
The present invention relates to an electroless plating solution, and more particularly, it relates to an electroless plating solution containing trivalent titanium ions for serving as a reductant.
2. Description of the Background Art
According to electroless plating, it is possible to homogeneously deposit a plating film of an arbitrary thickness not only on a metal base material of a noble metal such as gold or silver, copper or nickel but on a nonconductor base material of ceramic or plastic by supplying catalytic nuclei while employing a reductant such as formaldehyde, borohydride, hypophosphite or hydrazine. Therefore, electroless plating is not only applied to formation of a conductive film such as an electrode for an electronic component, for example, but is widely employed in various industrial fields of electronics, automobiles and the like, since it is possible to provide the as-formed plating film itself with various characteristics such as electrical resistance, magnetism, wear resistance and self lubricity.
In such electroless plating, however, metals which can be reduced and deposited with the aforementioned reductant are restricted to gold, silver, platinum group elements, copper, nickel and cobalt in general, and hence the application range thereof cannot be much enlarged. Phosphorus, boron, tungsten, iron and the like can be deposited by a eutectoid reaction in deposition of the aforementioned metals.
The assignee has proposed in Japanese Patent Application No. 1-328970 (1989) (Japanese Patent Application Laying-Open No. 3-191070 (1992)) a method which can deposit simple substances of antimony, indium, cadmium, lead, arsenic and zinc as well as alloys thereof in addition to the aforementioned gold, silver, platinum group elements, copper, nickel and cobalt, by noting trivalent titanium ions for serving as a reductant contained in an electroless plating solution and employing the same. Thus, it has been made possible to enlarge the range of elements which can be deposited by electroless plating.
However, the plating solution proposed in the aforementioned application has an irritating odor and a problem of safety in operation, since the same is ammonia-alkaline. Further, this plating solution may cause alteration of a portion of the substrate other than the surface to be plated. In addition, the pH value of the plating solution widely fluctuates during plating. Further, it is necessary to set the plating solution at high concentration and temperature levels, in order to improve the deposition rate. Thus, the aforementioned plating solution still has a number of problems to be solved.